This looks like old, unused code

I grepped through server logs for anything matching
/a/<domain>/migration, but didn't find anything.  The only code changes in the
past two years have been small updates from code-wide changes. Also, the
nature of the code seems like a one-off thing - "hit this endpoint to make
this thing happen once".  There are no references to `corehq.apps.migration` other than those removed here from `settings.py` and the main `urls.py`